With continuing remarkable growth in information technology and personal media devices, communication technology is taking center stage in the modern technological community. As consumers become more and more reliant on cell phones, PDAs, mobile televisions, personal navigation devices, personal media players, and a myriad of other devices employing communication systems, manufacturers are faced with growing pressures to build devices at lower cost with improved capabilities. A significant factor in the cost and performance of a communications device is the receiver. Incorporating cheaper and better-performing receivers into communication devices is critical for manufacturers aiming to compete in this space.
Generally, a communications system includes a transmitter communicating with a receiver over a communications channel. The transmitter modulates an original signal to create a carrier signal oscillating at a unique frequency. The carrier signal's unique frequency can define its channel in the system. A receiver can receive the carrier signal from an antenna or through a direct wire transmission. Before information contained in the carrier signal can be used, the carrier signal must be demodulated. The receiver's function is to demodulate the carrier signal and produce a signal that can be used by other components of the device to carry out the device's functions.
Complex receivers are a type of receiver in communications systems. Generally, the carrier signal is received in an analog portion of the receiver known as a mixer, where an in-phase signal component and a quadrature-phase signal component of the received signal are produced. The in-phase signal component and the quadrature-phase signal component can then be conveyed to other portions of the receiver for further demodulation and processing, which can take place in both analog and digital domains.
With the rapid evolution of semiconductor manufacturing and digital signal processing technologies, the digital portion of the complex receiver has, and continues to, rapidly become less costly to produce, more efficient, faster, and more precise. The analog portion of the complex receiver, particularly the mixer; however, continues to be relatively expensive and complicated, especially when high levels of performance are required.
What is needed, is a system and method for demodulation of a signal in a complex receiver where the mixer and the analog portion of the receiver is simplified and designed with less precision to reduce costs and complexity and the digital portion of the receiver is able to compensate for the simplification and imprecision of the analog side.
More specifically, a complex receiver receives a carrier signal that has been modulated from an original signal that comprised an in-phase and a quadrature-phase signal component. The complex receiver demodulates the carrier signal by extracting from the carrier signal an in-phase signal component of the original signal and a quadrature-phase signal component of the original signal. The demodulation is done by sending the carrier signal down separate paths of the complex receiver, the in-phase branch and the quadrature-phase branch. On the in-phase branch, the carrier signal is multiplied, in a mixer, by a COS signal that is generated by an oscillator at the frequency of the carrier signal. The resulting signal represents the in-phase (I) signal component of the original signal. On the Quadrature-phase branch, the carrier signal is multiplied, in a mixer, by a −SIN signal that is generated by passing the COS signal generated by the same oscillator through a phase shifter and shifting the phase by −90 degrees. The resulting signal represents the quadrature-phase (Q) signal component of the original signal.
The recovered I and Q signal components can exhibit imperfections known as an IQ-imbalance. Two sources of the IQ-imbalance are a gain differential between the I and Q signal components and an error in phase shift between the I and Q signal components. Namely, since the carrier signal travels down separate paths of the complex receiver to produce the I and Q signal components, differences in resistance and/or capacitance between the paths can create a difference in gain between the resulting I and Q signal components, known as a gain imbalance. Also, when the carrier signal is mixed with the COS and −SIN functions, an error in phase shift between the COS and −SIN functions will create an error in phase shift between the resulting I and Q signal components, known as a phase imbalance. The presence of an IQ-imbalance creates imperfections in the recovered signals and impairs a device's performance.
Methods have been developed for correcting the IQ-imbalance employing calibration signals, reference signals, and other means. However, these methods often require high-precision manufacture, complex architecture, and interdependence between components; all of which results in increased costs and decreased performance, reliability, and flexibility. What is needed is a mechanism for correcting the IQ-imbalance that does not depend on a calibration framework, that simplifies the structure of receivers, and that can be applied reliably and with minimal interdependency between components. As will be seen, the invention meets this need in an elegant manner.